Those processes of extraction involving the use of solvents may be divided into two categories, according to the causes that originate the difference in solubility: those cases in which such difference arises from physical causes and, those in which the difference is brought about by a chemical interaction between one or more of the diluted substances and one of the solvents employed.
Extraction involving the use of solvents is based on the co-existence of two liquid phases, one organic and the other aqueous, between which a given proportion of the diluted substance, e.g. metal, is distributed. There is a determined balance or equilibrium point to ascertain this proportion and such balance point is known as the distribution factor. The ratio in which the concentrations of the various diluted substances are found, in relation to the solvent media used, determines the distribution factor.
The separation of the substance or substances sought to be extracted depends upon the balance in which the two indicated liquid phases -organic and aqueous- are found, determined by the control held over chemical factors such as pH, concentrations, etc., on the basis of the volumetric relation between the relative quantities of both contacting phases. Should the recuperation achieved be small, this can be increased either by altering the previously indicated volumetric relation, or by adding new contact stages between both phases.
In the conventional solvent extraction practice, there exist two inseparable operations: the organic solvent loading and the organic solvent unloading. When the extraction steps proceed, the organic phase is loaded with the substance or substances, e.g. metallic, to be extracted from the original aqueous phase. This loading is always achieved in several and successive loading stages or contacts between the two phases till the organic phase is charged tending to the maximum thermodynamically permissible. The solvent extraction as an unit operation is still not concluded. The next successive contacts proceed between the loaded organic phase and a new aqueous phase, the stripping aqueous flow, which has distinct characteristics as the first one, in such a way that the organic phase is discharged, eluted, or stripped, and then returned to the first extraction stage, and so closing the organic circuit.
Three procedures to obtain this multiple contact in an industrial form are known for their efficiency:
(a) Co-current flow, in which the organic and the aqueous phases flow through the reactors, making contact in successive stages, in the same direction. Two circuits are used, one to load the organic (extraction) and another to unload it (stripping or elution). An outline of this procedure is shown in FIG. 1. Ac.sub.1 represents the flow of aqueous solution subject to the extraction process. Ex.sub.1, Ex.sub.2 and Ex.sub.3 define the extraction reactors through which aqueous solution Ac.sub.1 flows. Ac.sub.2 is the flow of the stripping aqueous solution, and El.sub.1 and El.sub.2 indicate the elution reactors through which the latter flows. ORG. denotes the solution or organic phase that flows successively through the extraction and elution reactors. The arrows show the direction of the flow in each case.
(b) Counter current flow, in which the phases flow in opposite direction along the reactors or contact stages. Loading and unloading circuits must likewise be used. This procedure has been outlined in FIG. 2, whose symbols are the same as those of FIG. 1. It may be noticed that the direction of the flow of the extraction and stripping aqueous phases is different, compared to the organic phase flow direction.
(c) Crossed flow, in which one phase flows along the reactors going through the successive stages and the other phase flows through each stage only once, "crossing" the other flow. This procedure also requires several loading units and several stripping units. FIG. 3 depicts such a procedure. The symbols are the same as those used in FIGS. 1 and 2.
Each system has its own advantages and deficiencies, which makes them appropriate for various industrial situations. The goal is to maintain a high driving force, proportional to the concentration difference of the element to be extracted in both phases.
The co-current system is useful for processes whose distribution factor is very high, thus making it very easy for the organic phase to receive the component in the aqueous phase.
In systems whose coefficient is lower, the organic and aqueous phases must be put into contact in such a way as to have the solutions come into contact through the most concentrated stage of one phase with the most diluted stage of the other phase. Thus, an adequate efficiency of the system is maintained.
The crossed flow system endeavors to have one "fresh" or recently unloaded organic phase make contact with each one of the successive stages, in such a way that the organic phase reloads decreasingly in each stage.
This system is very practical when a large quantity of the phase that loads the system is available, inasmuch as a great volume of this phase, with the given organic-aqueous proportion, is required for each stage.
Generally speaking, all systems in use try to load the organic phase to the maximum during the successive stages of extraction, and to unload the organic phase as much as possible in the successive stages of stripping, thus enabling use of the highest effective load of this phase.
This is done, with either one circuit or another in successive loading (or unloading) contacts, in order to arrive whenever possible at the balance point between the phases.
Under equivalent conditions, the indicated systems operate with the following virtues and defects.
Co-current: Requires many stages in order to reach high yields. It is limited by only one balance point provided by the relation between the input variables of the solution into the system. The maximum load is not rapidly attained in the organic phase.
Counter current: Requires a fewer number of stages. The organic phase is easily and rapidly loaded. It is limited by the successive balances reached in each stage.
Crossed flow: Is equivalent to one system of balance for each stage. It loads the organic increasingly of decreasingly according to the direction of the other. It requires many times more volume of organic phase than in the previous cases (in the event that the organic phase should cross the aqueous flow).
From the standpoint of maximum recovery of the element as of the aqueous phase, the co-current system presents the poorest condition in relation to the other two; the counter current, on making the less loaded organic phase contact the more so aqueous one and vice versa at the other extreme of the system, allows a good recovery, in equal number of stages. The crossed flow procedure may easily register recoveries of around 100% but with a very high volume of organic to treat the same aqueous volume.
The U.S. Pat. No. 3,193,381 to George et al shows two independent solvent extraction operations for the iron circuit (shown on the left side of the drawing). The first is a solvent extraction system to separate iron from the main flow (which contains also nickel and cobalt) using dioctyl phosphate (or other organic solvent) which is stripped with a high acid stripping solution. Then there proceeds the second solvent extraction circuit using a different organic solvent under other chemical conditions, which uses water as a stripping reagent. So, in the first operation iron is removed from the original solution (containing nickel and cobalt), and then proceeds the second operation starting with the stripping flow of the preceding process (this new flow is a high acid iron solution) and using another reagent in order to regenerate the acid and to put iron in the second stripping flow (water) which is discarded (without acid, economically). Additionally, in the George et al patent both solvent extraction circuits (for iron) are made with the normal counter current arrangement of aqueous and organic phase flows.